Update the new explorer advantages article to fit into the learn section

[pavel-fokin]

• Add new section to Learn section.

• Update usage of header tags.
• Generate cover image.
• Add related articles at the end.

[ihomp]

The link in the footer goes to /en/explorer-advantages—that seems not to exist.

[ihomp]

if the slug was changed, we need to update it in sitemap file too
no need to add redirect, as probably noone bookmarked it yet

[ihomp]

The link seems broken in the footer

[pavel-fokin]

@ihomp I fixed sitemap and footer link, please check. I'm planning to add proper link to the new Learn section in the following PR.